feat: full match expressions in bootstrap and selfhost

Lower match to if-else for literals, ranges, enum tags, and wildcards.
Fix bootstrap literal arms that always matched; port real arm AST/parser
and Lcx_LowerMatch to selfhost with last-expression return. Expand
pattern_matching example and add parse/use-after-move/double-mut golden
diagnostics. Selfhost-loop remains binary-identical.

bootstrap/c_backend.nim

@@ -401,7 +401,8 @@ proc emitExpr(be: var CBackend, node: HirNode): string =
     return "0"
 
   of hMatch:
-    return "0"  # TODO: match expression lowering
+    # Match should be desugared in hir_lower to if-else. Fallback: 0.
+    return "0"
 
   else:
     return "0"